In recent years, considerable effort has been expended in developing improved methods and apparatus for optically recording on and reading from a suitable optical media such as video discs and compact discs (CDs).
For example, Philips Technical Review, Volume 33, pages 187-193 and 197, and Philips Technical Review, volume 40, number 6, 1982, pages 151-164 provide examples of known optical recording technology.
Additionally, U.S. Pat. No. 4,107,528 (Silverman) discloses a system for directly transferring a preselected pattern of opaque and transparent areas in a first metal web to an underlying second web via optical means.
However, in order to initially record data onto the master web disc, complex apparatus and circuitry must be employed to ensure complete synchronism between the input data stream, speed of rotation of the disc, radial translation of the disc relative to the laser, and to control of the write laser etc.
U.S. Pat. No. 4,228,326 (Dakin et al) discloses a tracking control system for adjusting the angular velocity of an optical disc and the radial velocity of an optical transducer to be inversely proportional to the radius of the particular information track being recorded, whereby the track is moved at a constant linear velocity relative to the transducer for recording information of uniform density over the surface of the disc.
U.S. Pat. No. 4,546,914 (Winslow) teaches beam placement control in an optical disc recording system by means of a lead screw and translation drive.
The above discussed optical disc recording systems share a number of common requirements relating to encoding of data, data synchronization, disc formatting and data access and retrieval. Each of the above discussed systems provides for these requirements in different ways.
For example, the compact disc audio system (CD) described in the Philips Technical Review, Volume 40, encodes sound samples as a digital channel bit stream organized as "frames" which incorporate synchronization patterns. Such frames are recorded as a series of pits and lands, and the transition points between pits and lands are commonly known as "pit-edges". Patterns of pit-edges are formatted as a spiral track of pitch 1.6 .mu.m, which track is designed to be scanned at a constant track velocity of 1.25 m/s. The Digital signal is read by timing the track speed via a crystal so that the intervals between pit-edges may be measured. The compact disc audio system is a non-return-to-zero system in which each pit-edge (whether the start of a pit or a land) counts as one and each channel-bit-long length of track (Tc) is counted as zero. By way of contrast, the Philips LaserVision.TM. system discussed in Philips Technical Review volume 33, pages 187-193, also employs spiral tracks comprised of pits and lands, but in this case the data is stored in analogue form. Similarly, the disc recording systems disclosed in U.S. Pat. No. 4,228,326 (Dakin et al) and U.S. Pat. No. 4,456,914 (Winslow) which encompasses concentric tracks and a control system whereby each track is designed to be recorded and scanned at a constant linear velocity.
As discussed above, one fundamental disadvantage of such prior art systems in the formation of recordings is the requirement for precise synchronization of various subsystems, including, but not limited to, the following: the source data stream; the operation of a laser or other energetic beam; the turntable rotation speed; and the radial translation drive by which the laser and the object disc are moved radially relative to one another. De-synchronization of any subsystem relative to the other subsystems can result in erroneous data recording.
While such prior art has been widely employed in recording master discs such as those used in the mass production of compact disc recordings, it is complex and expensive. These disadvantages have made the creation of unique or custom recordings of data extremely difficult.
Numerous attempts have been made at simplifying the complex synchronization problems of prior art systems. Although some such attempts have resulted in technically and economically satisfactory systems for the creation of masters employed in the mass production of optical discs, such systems have been neither simple enough nor economical enough to warrant their application to the problem of unique production of compact disc recordings.
One alternative prior art approach reduces the complexity of the recording system by employing a prerecorded guide track on the object disc. According to this prior art approach, a two stage recording procedure is followed. First, the timing and format information is recorded on the object disc during the manufacturing process or, alternatively, when the object disc is formatted upon first installation in the system. Next, an information signal is recorded on the disc. Synchronization, track formatting, location and, some applications data addresses can be determined with comparatively simple, inexpensive and relatively reliable servosystems. Despite the theoretical promise of this prior art technique, difficulties have arisen in keeping the information signal recording process from interfering with or damaging the prerecorded guide track.
Additional prior art techniques have been developed for avoiding the inadvertent recording of an information signal over the guide track. One such technique provides for two mutually adjacent tracks, one reserved for information storage and one reserved for the prerecorded guide track. This approach has not been totally satisfactory because, despite the track separation, tracking error signals applied to scan the guide track may be affected by difracted light from the beam employed in recording and/or reproducing the information signal on an adjacent track.
In response to this problem, U.S. Pat. No. 4,754,339 (Nagai et al) discloses an information recording disc prerecorded with a considerably more precise guide track, including address and pseudo-address data which it is claimed overcomes such difficulties.
A further alternative approach is described by U.S. Pat. No. 4,494,226 (Hazel et al). According to Hazel et al, a three beam optical memory system is provided for use with a specially chosen preformatting of the optical disc. This system employs a read-before-write beam which scans the track ahead of a write laser to, among other things, avoid over-writing of previously recorded data. A disadvantage of this system is that the cost of preformatting the object disc increases the unit cost of the data stored.
It will be appreciated from the foregoing that there is a need for a system which can simplify the reliable production of optical recordings and do so at an economical cost.